The milk bombesin (MB) composition of bovine milk has been extensively investigated from the points of view of obtaining pure peptides and of atempting to characterized the biological properties of the milk extract. The purification efforts involved several manipulations of an acid extract of milk including: differential solubilization, ammonium sulfate precipitation, gel filtration, chromatography on ion exchange media, and reverse phase HPLC using a variety of columns and conditions. Smooth muscle contractility studies have suggested that one of the MB components of bovine milk may have biological properties similar to those of amphibian bombesin. However, there are multiple forms of MB in bovine milk extracts, and some of these do not appear to be biologically active. Furthermore, there are biologically active milk extract peptides which do not cross-react with the antiserum which is used to recognize MB; that antiserum recognizes the peptide sequence -Gly-Asp-Leu-Trp- (residues 5-8 of bombesin). Efforts are continuing to characterize the smooth muscle contractile profiles of milk extracts and to achieve final purification of several of the low-molecular weight range-MBs.